METEOROLOGIST JEFF HABY
Upper level lows are important to forecasting and can dramatically alter one's forecast. Upper level lows can occur
in association with a mid-latitude cyclone or may begin without the aid of a mid-latitude cyclone. Upper level
lows without the aid of a surface low can develop when air flows over a mountain range, in association with an
upper level short wave, or in association with a jet streak.
When analyzing a strong mid-latitude cyclone, some
common patterns can be noticed. One is that the trough associated with a mid-latitude cyclone tilts toward the
cold air (generally tilts to the northwest with height).
Therefore, the upper level low pressure (trough) in
association with a mid-latitude cyclone may be several 100 kilometers displaced from the surface low toward
the west or northwest. Since the forecast models
have a more difficult time initializing an upper level low than
a surface low, upper level lows can result in a busted forecast. The forecast models have a better vertical
resolution of the low levels of the troposphere as compared to the upper levels. In some mid-latitude cyclones,
the tilt of the mid-latitude cyclone will be enough to allow the upper level low to displace from the surface
What causes an upper level low? An upper level low is a region of
positive vorticity. This positive
vorticity can be caused by counterclockwise curvature around the
upper level trough and counterclockwise shear associated
with the speed shear of a jet streak. The circulation around an upper level low can build to the surface over
time. In these cases, two areas of low pressure will be noticed on the surface chart. These are sometimes
referred to as double-barrel low-pressure systems. Upper level lows can also decrease in intensity through
A huge forecasting problem is determining whether an upper level low will strengthen or weaken with time.
When nowcasting, they are best viewed on satellite imagery. Image by image they should
be monitored for intensity. When the
clouds brighten (become whiter) in association with the upper level low, that is an indication the upper
level low is strengthening.
If an upper level low does show on the analysis or forecast models it is best seen
at the 500 millibar level or 700 millibar level. Upper level lows have been responsible for bringing unexpected
heavy snows in the winter. The spin-up of vorticity in an upper level low causes the air to rise and cool.
Since the upper level low is tilted over the cold air, cold surface temperatures and upper level lifting combine
to produce wintry precipitation well behind (to the west or northwest) or the surface cold front. When a
mid-latitude cyclone begins to mature, watch for the development of the upper level low.